In fabricating integrated circuits, multiple patterns are projected, by means of exposure, each one by one from a mask or reticle into resist layers formed on a semiconductor wafer. Each of the exposures is usually followed by steps of processing the layer, which may relate to baking and developing the resist layer, etching the pattern formed in the resist further into an underlying layer, removing the resist, etc. such that a number of levels of the integrated circuit are sequentially formed.
With continuously increasing densities of structures within patterns, the resolution capability of exposure tools has been reached, which happens when structure widths become comparable to the wavelength of an exposure light beam, which is used to project the pattern onto the wafer. Accordingly, resolution enhancement techniques have been developed to push the dimensional limits in forming structures to even smaller structure widths, or pitches of periodic patterns. These techniques relate, for example, to the use of half-tone or alternating phase shift masks in an exposure, or to apply optical proximity corrections to the mask layout.
Critical patterns to be transferred from a mask to a semiconductor wafer are, e.g., contact hole patterns. Contact holes are generally formed to connect different layers of an integrated circuit and may include periodically arranged square-like openings, e.g., as in the case of dynamic random access memories (DRAM), where the contact holes have a particularly critical width with respect to the resolution limit of a respective exposure tool.
In comparison with line and space patterns, e.g., contact hole patterns are less suited for the alternating phase shift mask technique, although it has been shown in Schenker et al. “Alternating Phase Shift Masks for Contact Patterning” in: Optical Microlithography XVI, Proceedings of SPIE, vol. 5040 (2003) that an improvement with respect to conventional mask lithography may be accomplished. According to an analysis described in Schenker et al., additional constraints are placed on a lithographic projection step when tight pitches (i.e., large structure densities) have to be manufactured. These constraints particularly relate to accurate phase control, defect inspection and specific repair requirements.
Thus, the application of specific mask types, such as phase shift masks, or the application of specific features within the pattern, such as assist features, leads to considerable limitations particularly with respect to depth of focus (DoF), etc. Further, the application of specific mask types necessitates an extraordinarily high precision with regard to an etch process performed on a corresponding mask substrate.
In order to improve the process window when lithographically projecting contact hole patterns onto wafers, the use of so-called Bessel contacts has been proposed in Schellenberg et al., “Optimization of Real Phase Mask Performance”, 11th Annual BACUS Symposium on Photomask Technology, SPIE vol. 1604 (1991), page 274-296. According to that option, contact hole openings on the mask are provided with thin rim-like phase edges adjacent to the boundary of such openings. However, this option has only very limited application ranges and is further difficult to realize in mask manufacturing processes.
Another approach to improve process windows relates to pupil filtering, as described in Gräupner et al., “Solutions for Printing Sub 100 nm Contacts with ArF”, Optical Microlithography XV Proceedings of SPIE Vol. 4691, pp. 503-514 (2002). Therein, the formation of tri-tone contacts is suggested in the case only of large pitches (p>400 nm), while the application of pupil filtering, especially transmission filtering, within the pupil plane of the projection lens is not considered to improve the process window. Concurrently, the machine complexity and wafer throughput increases therefrom. As a consequence the approach of pupil filtering is also of limited use with respect to transferring contact hole patterns.
Another approach relates to focus drilling. A lithographic step is divided into a number of exposure steps, each step being performed with another focus offset of the wafer with respect to a focal plane of the projection lens. This method is also called “focus latitude enhancement exposure” (FLEX) and is disclosed, e.g., in Fukuda, H, et al. “Improvement of defocus Tolerance in a Half-Micron Optical Lithography by the Focus Latitude Enhancement Exposure Method. Simulation and Experiment”, J. Vac. Sci. Technol. B.7 (4) July/August 1989, P. 667-674. As is pointed out in Fukuda et al., the drilling or variation of focus is particularly promising with respect to contact holes. However, a problem arises with respect to the realization as a single exposure (throughput issues) and further the control of that exposure (loss of focus accuracy due to repeated auto focus adjustment for each exposure).
In literature, it has further been proposed to apply a y-tilt to the wafer stage, which results in a through-the-focus exposure (see, e.g., Lalovic, E et al. “Depth of Focus Enhancement by Wavelength Modulation: Can We RELAX and Improve Focus Latitude?”, Proceedings of 40th Interface Symposium Conference 2003), when the movement of a scanning in y-direction is adapted to the tilt. Disadvantageously, this proposal suffers from the limited dynamic leveling range of the exposure during scanning and a non-uniformity of the light intensity across the slit of a scanning apparatus. Further, the increase in depth of focus (DoF) is smaller than for, e.g., 2-3 exposures applied with a focus offset according to the FLEX-method.
It has further been suggested to mimic a focus offset by means of performing exposures with dual wavelengths of the laser instead of a single wavelength. The wavelength linearly scales with focus offset such that, instead of varying the height of the wafer stage, the laser of an exposure tool light source can be tuned to exhibit two wavelengths in one exposure, or a first wavelength in a first exposure and a second wavelength in a second exposure.
A prerequisite of this approach is that the projection optics reveal a chromatic aberration in length direction (CHL). However, disadvantages arise when pattern displacement and image deterioration occur over the scanner field by means of chromatic magnification that comes along with lens imperfections.
From the foregoing, it becomes clear that any known approach of driving sizes of structures in contact hole patterns nearer to the resolution limit suffers from one of: process window decrease, throughput reduction, severe constraints on mask manufacturing, constraints on exposure tool design, etc.